STTM devices are non-volatile memory devices that utilize a phenomenon known as tunneling magnetoresistance (TMR). For a structure including two ferromagnetic layers separated by a thin insulating tunnel layer, it is more likely that electrons will tunnel through the tunnel layer when magnetizations of the two magnetic layers are in a parallel orientation than if they are not (non-parallel or antiparallel orientation). As such, a magnetic tunneling junction (MTJ), typically comprising a fixed magnetic layer and a free magnetic layer separated by a tunneling barrier layer, can be switched between two states of electrical resistance, one state having a low resistance and one state with a high resistance. The greater the differential in resistance, the higher the TMR ratio: (RAP−Rp/Rp*100% where Rp and RAP are resistances for parallel and antiparallel alignment of the magnetizations, respectively. The higher the TMR ratio, the more readily a bit can be reliably stored in association with the MTJ resistive state. The TMR ratio of a given MTJ is therefore an important performance metric of an STTM.
For an STTM device, current-induced magnetization switching may be used to set the bit states. Polarization states of one ferromagnetic layer can be switched relative to a fixed polarization of the second ferromagnetic layer via the spin transfer torque phenomenon, enabling states of the MTJ to be set by application of current. Angular momentum (spin) of the electrons may be polarized through one or more structures and techniques (e.g., direct current, spin-hall effect, etc.). These spin-polarized electrons can transfer their spin angular momentum to the magnetization of the free layer and cause it to precess. As such, the magnetization of the free magnetic layer can be switched by a pulse of current (e.g., in about 1-10 nanoseconds) exceeding a certain critical value, while magnetization of the fixed magnetic layer remains unchanged as long as the current pulse is below some higher threshold associated with the fixed layer architecture.
For a pSTTM device, MTJs include magnetic electrodes having a perpendicular (out of plane of substrate) magnetic easy axis and can realize higher density memory than in-plane variants. Perpendicular magnetic anisotropy (PMA) can be achieved in the fixed magnetic layer through interfacial perpendicular anisotropy promoted by an adjacent layer during solid phase epitaxy. An anti-ferromagnetic layer or a synthetic antiferromagnetic (SAF) stack within an MTJ stack can further improve device performance by countering a fringing magnetic field associated with the fixed magnetic material layer.
Metallization employed as an electrode over which an MTJ stack is formed can impact MTJ performance. For example, efforts to achieve low electrical resistivity for the electrode and/or compatibility with conventional metal-oxide-semiconductor (MOS) transistor integrated circuitry (IC) can reduce MTJ fixed layer stability and/or cause TMR loss. pSTTM architectures including both MOS transistor-compatible electrode material and a high performance MTJ are therefore advantageous.